Isotactic polypropylene resins are useful for many applications. The relatively high end-use temperature, and high modulus of the material are two features which contribute strongly to its utility among polyolefins. Modulus refers to the shear modulus value of a given resin obtained through dynamic mechanical testing in accordance with ASTM D4065. For purposes of this invention, service temperature is defined as the end-use temperature of the article, film, sheet, or fiber produced from the polypropylene resin. The Heat Distortion Test (HDT), also referred to as Heat Deformation Test or Heat Deflection Test, is widely used to define the service temperature of thermoplastics. In this test, a weight is hung from a cantilevered molded bar held in a chamber and the temperature of the chamber is raised at a set rate until the bar droops a given amount. This test is conducted in accordance with ASTM D648 at either 66 psi (455 kPa) or 264 psi (1820 kPa) specific load. The HDT value is a measure of the resistance to deformation of a material under the influence of stress at an elevated temperature. HDT value is an indication of the highest temperature at which a plastic material retains acceptable integrity for commercial utility. HDT values referenced herein are relative to the 66 psi (455 kPa) specific load test.
In order to maximize the utility of polypropylene resins, it is desirable to design the resin to have as high a modulus and HDT value as possible. Typically, for conventional polypropylene resins maximizing the modulus and HDT value also maximizes the melting point of the resin. For melt processing, such as done for injection molded articles, fabrication of the polypropylene occurs at temperatures above the melting point. The properties of polypropylene can be enhanced by orientation at temperatures slightly below the crystalline melting point. Textile fibers, uniaxially, and biaxially-oriented films are examples of products which benefit from such an orientation process. For orientation processing, the temperature of the polypropylene is held at a temperature slightly below the melting point. Generally, in either processing, the high melting point of the resin requires the use of high temperatures for the processing equipment. This is undesirable because heating the resin to the elevated temperature prior to processing, and then cooling it down afterward takes additional time which affects production rates and economics. Additionally, this is undesirable because energy consumption of the process suffers.
According to Spalek, et.al., at a presentation made at the Metallocene Conference, Houston, Tex., May, 1993, the relationship between melting temperature and room temperature modulus are different for metallocene and conventional polypropylenes. Spalek, et.al., disclosed that metallocene homopolypropylene have higher room temperature modulus values than conventional polypropylene with comparable melt flow rate (MFR) and melting point (MP) values. In durable goods and high performance packaging applications, one is interested not only in room temperature modulus, but also in how this modulus responds to elevated temperatures.
It would be highly desirable to have a polypropylene resin having high modulus and HDT values, but which could be processed at a temperature below that required for a conventional polypropylene with comparable modulus and HDT values. Generally, this means having a resin with a high modulus and HDT value, but with a low melting point. For conventional isotactic homopolymers of propylene, the crystalline melting point is in the range of about 160.degree.-165.degree. C. and the HDT value is in the range of about 90.degree.-105.degree. C. For these conventional isotactic homopolymers, the orientation process generally occurs in a temperature range of about 135.degree.-155.degree. C. Thus, orientation temperatures for conventional polypropylene are about 40.degree.-50.degree. C., or more, above the HDT value. It would be highly desirable to process isotactic polypropylene resin at lower temperatures than currently known without compromising HDT value, modulus or other important properties of the resulting product.